(1) Field of the Invention
This invention relates to a D/A converter and semiconductor device and, more particularly, to a D/A converter and semiconductor device for outputting analog voltages which correspond to digital values and which vary around half of power supply voltage.
(2) Description of the Related Art
With the progress of digital information processing, at present an analog quantity is converted into a digital quantity with an A/D converter to perform digital processing. On the other hand, a digital quantity obtained by digital processing is converted into an analog quantity with a D/A converter to control. A D/A converter may be used in an analog delayed locked loop (DLL) in a dynamic random access memory (DRAM) (see, for example, Japanese Unexamined Patent Publication No. 2002-305449, paragraph nos. [0023]–[0026] and FIG. 2).
FIG. 3 is a circuit diagram of a conventional 8-bit D/A converter.
As shown in FIG. 3, an 8-bit D/A converter comprises a resistor circuit 101, a switching circuit 102, a decoder circuit 103, a conversion circuit 104, and an amplifier 105.
The resistor circuit 101 includes resistors R101a, R101b, R101c, . . . , R101m, and R101n connected in series. The resistor R101a is grounded and the resistor R101m is connected to a power supply of Vdd. 256 voltages which make up an arithmetic progression are taken from the resistors connected in series in the resistor circuit 101.
The switching circuit 102 includes switches SW101a, SW101b, SW101c, . . . , SW101m. Voltage outputted from the resistor circuit 101 is inputted to one end of each of these switches. The other ends of these switches are connected to one another.
The decoder circuit 103 has 256 signal lines on the output side. These signal lines are connected to the switches, respectively, in the switching circuit 102 to control them so that they will be turned on/off. The 8-bit digital signal “D0, D1, . . . D6, D7” is inputted to the decoder circuit 103. The decoder circuit 103 decodes the digital signal inputted and outputs a signal to a signal line corresponding to the digital value of the digital signal. As a result, a voltage corresponding to the digital value will be outputted from the switching circuit 102.
The conversion circuit 104 converts the voltage which is outputted from the switching circuit 102 and which corresponds to the digital value into a voltage which varies around reference voltage and outputs it. The conversion circuit 104 includes an operational amplifier 104a and resistors R102 and R103.
The amplifier 105 outputs the reference voltage to the conversion circuit 104. The amplifier 105 outputs a voltage equal to half of Vdd to the conversion circuit 104 as the reference voltage. Therefore, the conversion circuit 104 can output voltages which vary around half of Vdd and which correspond to digital values. That is to say, the conversion circuit 104 can output voltages which correspond to digital values and which range from 0 V to Vdd.
The amplifier 105 outputs the voltage equal to half of Vdd generated by resistors to the conversion circuit 104 via a voltage follower.
FIG. 4 is a circuit diagram of the amplifier 105 shown in FIG. 3.
As shown in FIG. 4, the amplifier 105 includes resistors R104 and R105 and an operational amplifier 106.
The resistors R104 and R105 are connected in series. The resistor R105 is connected to the power supply of Vdd and the resistor R104 is grounded. A non-inverting input terminal of the operational amplifier 106 is connected to a point where the resistors R104 and R105 are connected. An inverting input terminal of the operational amplifier 106 is connected to the output side. The operational amplifier 106 forms the voltage follower.
If the resistance values of the resistors R104 and R105 are the same, then a voltage equal to half of the power supply voltage Vdd will be outputted from the operational amplifier 106.
Voltage outputted from the operational amplifier 104a shown in FIG. 3 will now be calculated from equations. The following equation (1) holds.(Vo−Vi)/r103=(Vi−1/2Vdd)/r102  (1)
where Vi is voltage outputted from the switching circuit 102, Vo is voltage outputted from the operational amplifier 104a, r102 is the resistance value of the resistor R102, and r103 is the resistance value of the resistor R103.
By changing equation (1), the following equation (2) can be derived.Vo−Vi=r103/r102(Vi−1/2Vdd)Vo=(1+r103/r102)(Vi−1/2Vdd)+1/2Vdd  (2)
As indicated by equation (2), voltage Vo outputted from the operational amplifier 104a varies around half of the power supply voltage Vdd.
Another example of conventional D/A converters will now be described.
FIG. 5 is a circuit diagram of another example of conventional D/A converters.
As shown in FIG. 5, a D/A converter comprises resistor circuits 111a, 111b, 111c, etc., switching circuits 112a, 112b, 112c, etc., decoder circuits 113a, 113b, 113c, etc., conversion circuits 114a, 114b, 114c, etc., and an amplifier 115.
The D/A converter shown in FIG. 5 includes a plurality of D/A converters each of which is the same as that shown in FIG. 3. That is to say, the D/A converter shown in FIG. 5 is a multichannel D/A converter in which a plurality of digital signals are converted into analog signals. In this case, a voltage equal to half of power supply voltage Vdd is also generated by the amplifier 115. The reference voltage generated by the amplifier 115 is inputted to the plurality of conversion circuits. As a result, voltages which correspond to digital values and which are outputted from the plurality of switching circuits will vary around the reference voltage.